PURPOSE: To evaluate the incidence and characteristic findings of pineal cyst incidentally detected on magnetic resonance(MR) imaging. MATERIALS AND METHODS: Brain MR images obtained in 2432 patients were retrospectively reviewed to determine the incidence and MR findings of pineal cysts, which were evaluated according to their size, shape, location, signal intensity, interval change, contrast enhancement and mass effect on adjacent structures. RESULTS: Cysts were encountered in 107(4.4 %) of 2432 patients evaluated. Their size ranged from 1 x 1 x 1 to 15 x 8 x 9(mean, 5.97 x 3.87 x 4.82)mm. All were spherical(n=53) or oval(n=54) in shape. Their margin was smooth and they were homogeneous in nature. On T1-weighted images, the cysts were seen to be hyperintense(n=57) or isointense(n=50) to cerebrospinal fluid, but less so than brain parenchyma. T2-weighted images showed them to be isointense(n=51) or hyperintense(n=56) to cerebrospinal fluid. The cysts were centrally located in 65 cases and eccentrically in 42. Compression of the superior colliculi of the tectum was demonstrated in 17 cases(15.9 %). No patients presented clinical symptoms or signs related to either pineal or tectal lesions. Peripheral enhancement around the cyst after Gd-DTPA injection was demonstrated in 51 cases ( 1 00 %). Follow-up examinations in 19 cases demonstrated no interval change. CONCLUSION: The incidence of pineal cysts was 4.4%. The MR characteristics of simple pineal cysts include: (1) an oval or spherical shape, (2) a smooth outer margin and homogeneous nature, (3) isosignal or slightly high signal intensity to cerebrospinal fluid on whole pulse sequences, (4) ring enhancement after contrast injection,(5) an absence of interval change, as seen during follow up MR study. These MR appearances of pineal cysts might be helpful for differentiating them from pineal tumors.